The present invention relates to a method of producing a thermal head. Particularly, this invention relates to the structure of a thermal head and a method of producing the thermal head, which offer high recording quality with improvement in the structure of a recording section for recording on a recording medium inserted between a platen and the recording section, for prevention of shifting of the recording section from a correct position to be recorded.
Thermal heads have an important place in the fields of OA (Office Automation)-devices and fax machines. A thermal head is made up of heating resistors arranged on a substrate. The heating resistors generate heat for recording on heat-sensitive papers or films of type-setting developing paper. Several developments have been in progress for their low noise level and low running cost. Thermal heads have recently attracted attention as being used for video printers, for example.
FIG. 1 is a plan view of an overall structure of a conventional thermal head, partially cut away in several stages for easy understanding. FIG. 2 is an enlarged cross sectional view taken on arrows Xxe2x80x94X of the thermal head shown in FIG. 1. In these drawings, the thermal head is provided with an aluminum substrate 1 formed with long narrow flat surface and having thickness for certain rigidity, a ceramic substrate 2 formed on a main surface of the aluminum substrate 1 and having a thickness of 0.5 to 1.0 mm, thermal resistance and relatively large coefficient of heat transfer, and several integrated circuits 8 (abbreviated to IC hereinafter) aligned in row at the side of the ceramic substrate 2. Formed on the center portion of the ceramic substrate 2 in the width direction in this structure is a glaze layer 3 as a heat-retention layer having a semi-gabled transversal surface formed in the longitudinal direction. A heating-resistor layer 4 is formed on the surface of the ceramic substrate 2 including the glaze layer 3.
The glaze layer 3 is mainly made of glass. The heating-resistor layer 4 includes an alloy, such as, Ta-SiO and made by a thin-film forming method, such as, sputtering. A conductive layer of aluminum, for example, is formed on the surface of the heating-resistor layer 4, and further, a common electrode 5 and separated electrodes 6 are formed thereon by photo-engraving. The common electrode 5 is formed like teeth of a comb on the one side of the glaze layer 3 from the center thereof. Several electrodes 6 are formed at the IC 8 side like strips aligned like teeth of a comb. The tips of the common electrode 5 and those of the separated electrodes 6 face each other at the summit of the semi-gabled glaze layer 3 with a predetermined gap. Connected to the other edge of each separated electrode 6 is a lead wire 9 extended from the IC 8. A heat- and abrasion-resistant protective-film layer 7 is formed by, for example, sputtering, over the surfaces of the common electrode 5 and the electrodes 6.
In this structure, a recording section 10 covers the summit of the protective-film layer 7 formed over the glaze layer 3 and the surrounding, as a thermal head having resolution of 220 to 400 d/i (the number of dots per inch).
The thermal head described above is provided with the recording section 10 at the portion corresponding to the summit of the glaze layer 3, thus having the common electrode 5 and the separated electrodes 6 with the tips of comb-like teeth facing each other over the center summit. The positions of the tips facing each other are sometimes shifted from the correct positions due to incomplete production process. This problem is discussed with respect to FIGS. 3 to 5.
The glaze layer 3 is usually formed with a semi-gabled transversal surface as illustrated in FIG. 3(a) by screen printing, for example. As shown in FIG. 3(b), the heating-resistor layer 4 and the conductive layer are stacked thereon. The photo-engraving is applied to the conductive layer to form the common electrode 5 and separated electrodes 6 with a gap 10A between the tips thereof. In the process of forming the glaze layer 3 on the ceramic substrate 2, as illustrated in FIG. 4, several glaze layers, for example, glaze layers 31 and 32 are simultaneously formed on a main surface of one ceramic substrate 2 followed by cutting on a dashed line Y to form a substrate (glazed ceramic substrate) having two glaze layers.
As described, since the conventional thermal head has a semi-gabled cross section, shifting of portions (recording section) at which electrodes facing each other if occur during patterning causes variation in height of the recording section from the substrate surface. Moreover, in formation of several glazed layers, wrinkles formed on a plate during screen printing could lower accuracy in straightness, thus causing undulation Z on a part of the glaze layer 2. Due to these problems, even photoengraving, a highly accurate electrode forming step, is applied to the conductive layer, as illustrated in FIG. 5, the recording section 10, which has to be located on the summit of the semi-gabled structure, is shifted from the summit, thus lowering printing quality.
A purpose of the present invention is, for solving the problems discussed above, to provide a thermal head and a method of producing the thermal head, by protecting a recording section from being shifted in height from a substrate, for attaining a higher printing quality and reliability and also a simplified production process.
Another purpose of the present invention is to provide a thermal head and a method of producing the thermal head, by protecting a recording section from being shifted which may otherwise occur due to shifting of a heating-resistor layer and separated electrodes caused by undulation of a glaze layer.
In order to attain the purposes, the thermal head according to the first aspect of the present invention includes a longitudinal substrate; a heat-retention layer made of a heat-retaining material, having at least a sticking-out section lying on one main surface of the substrate in a longitudinal direction at a constant width; a heating-resistor member made of a resistive material, formed at least on the sticking-out section of the heat-retention layer at a predetermined thickness; a common electrode provided as touching the heating-resistor member; a plurality of separated electrodes provided as facing a tip of the common electrode with a gap, at least an edge of each separated electrode touching the heating-resistor member, another edge of each separated electrode being connected to a driver circuit; and a protective layer formed on the heating-resistor member, a heating-resistor member portion provided on the gap between the common electrode and the separated electrodes functioning as a recording section, wherein the sticking-out section is formed in straight in the longitudinal direction of the substrate and has a almost flat summit surface, the recording section being formed on the flat summit surface.
In the thermal head according to the first aspect, the common electrode and the separated electrodes may be formed as rising from a side edge of the substrate in the longitudinal direction to a side face of the sticking-out section of the heat-retention layer and covering a side edge, in the longitudinal direction, of the heating-resistor member formed on the summit surface.
In the thermal head according to the first aspect, the heat-retention layer may have a skirt section formed on the main surface of the substrate at a almost constant thickness, the sticking-out section being formed on the skirt section as sticking out therefrom.
In the thermal head according to the first aspect, the heating-resistor member may be formed within a summit surface of the sticking-out section.
In the thermal head according to the first aspect, the separated electrodes and the common electrode may be formed as touching the heating-resistor member at a side face of the sticking-out section, summit surfaces of the separated electrodes and the common electrode and a summit surface of the heating-resistor member of the recording section forming an almost continuous plane.
Moreover, the method of producing a thermal head according to the second aspect of the present invention includes the steps of: a step of applying a heat-retaining material on a main surface of a substrate, thus forming a heat-retention layer; a step of applying a resistive material on a surface of the heat-retention layer, thus forming a heating-resistor layer; a step of forming a sticking-out section lying in straight, in a longitudinal direction, on the heating-resistor layer formed on an almost flat surface of the heat-retention layer by etching the heating-resistor layer and the heat-retention layer; a step of forming a common electrode, an edge thereof touching the heating-resistor member, and a plurality of separated electrodes provided as facing tips of the common electrode with a gap, at least one edge of each separated electrode touching the heating-resistor member; and a step of forming a protective layer at least on the heating-resistor member.
In the method of producing a thermal head according to the second aspect, it is preferable that the sticking-out section forming step has the step of forming the sticking-out section lying in straight, having a flat summit, including the heating-resistor member on the flat surface of the heat-retention layer by etching the heat-retention layer and the heating-resistor member stacked on the main surface of the substrate, and the electrode forming step has the step of forming the common electrode and the separated electrodes aligned like a row of teeth of a comb, the common electrode and the separated electrodes being provided from both sides of the sticking-out section to one main surface of the substrate, side faces of the heat-retention layer and a surface of the heating-resistor member, the teeth facing each other with a predetermined gap being connected to the heating-resistor member.
In the method of producing a thermal head according to the second aspect, it is preferable that the sticking-out section forming step has the step of forming the sticking-out section lying in straight, having a flat summit, over the main surface of the substrate, including the heating-resistor member on the almost flat surface of the heat-retention layer, the sticking-out section having a skirt section on both sides thereof in its cross sectional shape by etching the heat-retention layer and the heating-resistor member, and the electrode forming step has the step of forming the common electrode and the separated electrodes aligned like a row of teeth of a comb, the common electrode and the separated electrodes being provided from both sides of the sticking-out section to the skirt section of the sticking-out section, side faces thereof, and a surface of the heating-resistor member, the teeth facing each other with a predetermined gap being connected to the heating-resistor member.
The method of producing a thermal head according to the third aspect of the present invention includes the steps of: a step of applying a heat-retaining material on a main surface of the substrate, thus forming a heat-retention layer; a step of applying a resistive material on a surface of the heat-retention layer, thus forming a heating-resistor layer; a step of forming a sticking-out section lying in straight, in a longitudinal direction, on the heating-resistor layer formed at least on a flat summit surface of the heat-retention layer by etching the heat-retention layer; a step of forming a common electrode and a plurality of separated electrodes, edges of the electrodes being aligned like teeth of a comb and connected to the heating-resistor member, formed on the main surface of the substrate or the heat-retention layer in a direction from both sides of the sticking-out section to a side edge of the main surface of the substrate in a longitudinal direction, on the main surface of the substrate or another edge of each electrode being located in a side edge of the heat-retention layer in a longitudinal direction thereof, the common electrode and the separated electrodes being provided on at least a flat surface of the sticking-out section so that the common electrode and the separated electrodes face each other with a predetermined gap; and a step of forming a protective layer on the heating-resistor layer.
In the method of producing a thermal head according to the third aspect, it is preferable that the sticking-out section forming step has the step of forming the sticking-out section, lying in straight on the flat summit surface of the heat-retention layer, having an almost flat summit surface and a skirt section a cross section thereof being flat on both sides thereof, by etching the heat-retention layer, the heating-resistor member forming step has the step of applying the resistive material over the substrate from the skirt section of the heat-retention layer to the sticking-out section at an almost constant thickness, and the electrode forming step has the step of the forming the common electrode and the separated electrodes aligned like a row of teeth of a comb, the common electrode and the separated electrodes being provided from both sides of the sticking-out section to the skirt section and side faces of the sticking-out section, and a surface of the heating-resistor member, the teeth facing each other with a predetermined gap being connected to the heating-resistor member.
In the method of producing a thermal head according to the third aspect, it is preferable that the sticking-out section forming step has the step of forming the sticking-out section, lying in straight on the flat summit surface of the heat-retention layer, by etching the heat-retention layer, the heating-resistor member forming step has the step of applying the resistive material over the main surface of the substrate at an almost constant thickness except the entire sticking-out section of the heat-retention layer and a portion where the sticking-out section has been formed, and the electrode forming step has the step of the forming the common electrode and the separated electrodes aligned like a row of teeth of a comb, the common electrode and the separated electrodes being provided from both sides of the sticking-out section to the main surface of the substrate, side faces of the sticking-out section, and a surface of the heating-resistor member, the teeth facing each other with a predetermined gap being connected to the heating-resistor member.
The thermal head and the method of producing the thermal head according to each aspect provides the heat-retention layer of high straightness formed on the ceramic substrate with the sticking-out section of the glaze layer and also the recording section formed, between the tips of the common electrode and the separated electrodes, on the flat summit surface of the sticking-out section, thus achieving precise control of height of the recording section from the substrate for higher printing quality and reliability.